The present invention relates to a control valve for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a device for decreasing noise produced in a pressure control valve provided in a hydraulic circuit of the control system.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
The line pressure control valve disclosed in U.S. Pat. No. 4,467,674 is adapted to be applied with an elastic force of a spring dependent on the transmission ratio at one end of the spool, and with pitot pressure at the other end to generate line pressure. The line pressure is set to a comparatively high value depending on the transmitted torque and changed in a wide range in dependence dependency on the transmission ratio. Accordingly, the line pressure control valve is arranged to directly control the flow of oil from an oil pump so as to control the line pressure. However, fluctuation of the pump oil pressure causes the valve to vibrate, thereby generating noise. The noise become larger when the temperature of the oil is low.
One way of reducing the noise is to increase the volume of a passage from the pump to the valve, for example, by providing an accumulator communicated with the passage. However, the volume of the accumulator is limited in order to meet the requirement for a compact valve. Another is to reduce the cross-sectional area of the passage so as to increase the resistance. However, when the valve is manufactured by a metal mold, it is difficult to form a sufficiently small passage, due to the fragility of a casting pin.